Engineers usually face the difficulty of making careful choices between one good and another in terms of strength, ductility, and electrical conductance in nanostructured metals. The emergence of polycrystalline Cu with ultrafine grains and the included nanotwins supplies an ideal solution because such materials owe high strength, high electrical conductance, and intermediate ductility. We answer in this Brief Report where the strength of such materials maximizes and how it depends on grain size. Based on the competitive plastic deformation mechanisms-the strengthening mechanism by inclined dislocations (with respect to twin planes) and detwinning-induced softening-we find that the critical twin thickness where the strength maximizes is proportional to d(1/2) and the maximum strength is proportional to d(-1/2).